3 Answers
3

Somewhat related to Shalom's answer, the Lubavitcher Rebbe zt"l explains (Likkutei Sichos, vol. 25, pp. 235ff) that the miracle of the oil (a spiritual victory) overshadows the miracle of beating the Greek armies (a physical one) - because the latter was itself part of a larger spiritual war: they tried to make us "forget Your Torah and abandon the decrees of Your Will"; ordered the Jews to "write on the horn of an ox that you have no portion in the G-d of Israel"; etc.

So when we want to mention and describe the miracle of the military victory, we have to omit mention of the miracle of the oil. Conversely, when the Gemara explains מאי חנוכה, it mentions the victory only incidentally.

one of my favorite answers in Ner l'meah, about why 8 days, when the miracle was really only 7 days: Whenever we light oil and it burns, it is a natural "everyday" miracle, which sometimes requires supernatural miracles to focus our attention on the mundane ones.
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JeremyDec 2 '10 at 15:25

A wonderful answer to this question us given in the sefer Tal Oros by Rabbi Yehudah Tuvia Guttentag (Tavyomi). The full text in English can be found here, but the following excerpt contains the essential points:

After the great victory of
Yehudah the Maccabee against the many and strong Greek troops and over
the multitude of Hellenists, and after the purification of the
sanctuary, the rebuilding of the altar, the miracle of the flask of oil
and the celebration of the re-inauguration for eight days, Yehudah the
Maccabee and his Beis Din, the Great Beis Din that was called at that
time “the Beis Din of the Chashmonoim” (see Avodah Zorah 36),
they wanted to make a remembrance of these salvations and wonders, and
to establish these days of Chanukah as a festival for the generations
with Hallel and thanksgiving.

However, on this point there was a confusing problem; on what
to substantiate and base this festival? For which remembrance to
establish it? If to commemorate the great victory of the few against
the
many, and the weak against the strong - this would not be in accordance
with the rules of the Torah, because a festival with Hallel and
thanksgiving can only be fixed for a completely unnatural
miracle, as mentioned above. If to commemorate the miracle of the flask
of
oil that only contained enough oil to light for one day, but by a
miracle they managed to light from it for eight days, - this is an
‘inside miracle’, a miracle which occurred only in the sanctuary, in
the
presence of a handful of Kohanim whose day of service it was, or whose
week it was, in a place where it is totally forbidden for non-Kohanim
to enter. If in earlier, very righteous generations little importance
and value were given to ‘inside’ miracles like these, all the more so
in
that generation where there were numerous Hellenists. Certainly the
general population would not believe in this miracle; they would have
no regard for it and would not celebrate any festival in order to
remember
it.

After great deliberation, Yehudah the Maccabee and his Beis
Din
came to a decision: they would establish this festival of Chanukah with
Hallel
and thanksgiving and candle lighting, but without giving a
reason and substantiation for establishing this new festival, similar
to
other decrees and safeguards that were sometimes issued without
explanation. Like the gemara Avodah Zorah 30 says, “When they made a
decree
in Eretz Yisrael they did not reveal the reason for twelve months,
perhaps somebody might disagree with the reason, and the people would
come to make light of the decree”, and thus undermine the
decree.

With
this the Beis Din of the Chashmonoim have satisfied everybody. Because
those
who knew the Torah, who were expert in its laws and knew that a
festival with Hallel and thanksgiving
is only established for a non-natural miracle, they would trust the
testimony
of the Kohanim in the Beis Hamikdosh, amongst whom were certainly some
of the great Beis Din. They would not doubt even for a moment the
miracle of the flask of oil. The general populace, in the absence of an
explicit reason, would think that the festival was established as
a remembrance of the salvations and victories, and the lighting of
candles is to remember the renewed lighting of the menorah in the Beis
Hamikdosh. Let them think what they will, as long as they observe the
festival with all the its commandments, according to Halachah.

This
corresponds to the text of Al Hanissim. There, also, everything is
mentioned in a vague style - the
wars, the victories, together with the purification of the sanctuary
and
the lighting of the candles, which is based on the miracle of the flask
of oil. But this miracle is not
mentioned explicitly, because this prayer is for everybody, and
therefore they intentionally concealed this reason of the ‘inside’
miracle because of
ignoramuses.